Organic Light Emitting Diodes (OLEDs) have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of light-emitting elements. The light-emitting elements are typically arranged in two-dimensional arrays with a row and a column address for each light-emitting element and having a data value associated with each light-emitting element to emit light at a brightness corresponding to the associated data value. However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from non-uniformities in the light-emitting elements. These non-uniformities can be attributed to both the light emitting materials in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements.
A variety of schemes have been proposed to correct for non-uniformities in displays. U.S. Pat. No. 6,081,073<entitled “Matrix Display with Matched Solid-State Pixels” by Salam granted Jun. 27, 2000 describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,414,661 B1 entitled “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time” by Shen et al issued Jul. 2, 2002 describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. The compensation system is best used after the display device has been calibrated to provide uniform light output. This patent provides a means for correcting the non-uniformities through the use of a look-up table. However, this approach does not reduce variation and reductions in bit-depth for the various pixels in the display and requires a large lookup table and complex calculation and circuit to implement.
U.S. Pat. No. 6,473,065 BI entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan issued Oct. 29, 2002 describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, this approach uses complex and large electronic means to implement, and also suffers from reduced and variable bit-depth in display gray-scale.
Other techniques rely upon complex sensing and driving circuitry to provide uniformity correction. For example, U.S. 20020030647 entitled “Uniform Active Matrix OLED Displays” by Hack et al published Mar. 14, 2002 describes such a technique. In this design, an active matrix display comprises an array of pixels, each pixel including an organic light emitting device and at least one thin film transistor. A uniformity correction circuit that is capable of producing a selected pixel brightness is connected to the array of pixels. The uniformity correction circuit is capable of maintaining the brightness of the pixels in a range that does not vary, for example, by more than about 5–10% from their selected brightness values. In other examples, improved uniformity is achieved through complex pixel driving circuits in each pixel. For example, see EP0905673 entitled “Active matrix display system and a method for driving the same” by Kane et al published Mar. 31, 1999. These approaches can unfavorably reduce the area in the OLED display available for emitting light, reduce manufacturing yields, and are subject to uniformity variation in the pixel circuits themselves.
There is a need, therefore, for an improved method of providing uniformity in an OLED display that overcomes these objections.